# std::numeric_limits<T>::quiet_NaN

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< cpp‎ | types‎ | numeric limits

static T quiet_NaN() throw(); |  | (until C++11)
static constexpr T quiet_NaN() noexcept; |  | (since C++11)

Returns the special value "quiet not-a-number", as represented by the floating-point type `T`. Only meaningful if std::numeric_limits<T>::has_quiet_NaN == true. In IEEE 754, the most common binary representation of floating-point numbers, any value with all bits of the exponent set and at least one bit of the fraction set represents a NaN. It is implementation-defined which values of the fraction represent quiet or signaling NaNs, and whether the sign bit is meaningful.

### Return value

`T` | std::numeric_limits<T>::quiet_NaN()
/* non-specialized */ | T()
bool | false
char | ​0​
signed char | ​0​
unsigned char | ​0​
wchar_t | ​0​
char8_t (since C++20) | ​0​
char16_t (since C++11) | ​0​
char32_t (since C++11) | ​0​
short | ​0​
unsigned short | ​0​
int | ​0​
unsigned int | ​0​
long | ​0​
unsigned long | ​0​
long long (since C++11) | ​0​
unsigned long long (since C++11) | ​0​
float | implementation-defined (may be NAN)
double | implementation-defined
long double | implementation-defined

### Notes

A NaN never compares equal to itself. Copying a NaN may not preserve its bit representation.

### Example

Several ways to generate a NaN (the output string is compiler-specific):

Run this code

    #include <iostream>
    #include <limits>
    #include <cmath>

    int main()
    {
        std::cout << std::numeric_limits<double>::quiet_NaN()     << ' ' // nan
                  << std::numeric_limits<double>::signaling_NaN() << ' ' // nan
                  << std::acos(2)    << ' '   // nan
                  << std::tgamma(-1) << ' '   // nan
                  << std::log(-1)    << ' '   // nan
                  << std::sqrt(-1)   << ' '   // -nan
                  << 0 / 0.0         << '\n'; // -nan

        std::cout << "NaN == NaN? " << std::boolalpha
                  << ( std::numeric_limits<double>::quiet_NaN() ==
                       std::numeric_limits<double>::quiet_NaN() ) << '\n';
    }

Possible output:

    nan nan nan nan nan -nan -nan
    NaN == NaN? false

### See also

has_quiet_NaN [static] | identifies floating-point types that can represent the special value "quiet not-a-number" (NaN) (public static member constant)
signaling_NaN [static] | returns a signaling NaN value of the given floating-point type (public static member function)
nannanfnanl (C++11)(C++11)(C++11) | not-a-number (NaN) (function)
isnan (C++11) | checks if the given number is NaN (function)

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